"Brushed" Stacking / processing artifact - nature or software ??

[Craney]

Evening all,

Just going through some old data-sets whilst the clouds rule the skies and I have found an artifact that is appearing on my DSLR  (600D modded)  stacked images.

Here is the evidence....

Light sub (120s ISO 800)

Stacked image below   Left one  straight out of APP (  ...80 subs......  no bias or darks or flats on this run but the effect still shows up when I include them.....)

Stacked image below   Right one...... after various magics in PS  has been applied.....  Gradient exterminator ..... stretching routines  etc......

Rosette btw... but some of you probably guessed that already..

so if you zoom in on the right hand one  (the one stacked in APP and processed in PS),  you get this brushed effect.... 

There is no evidence of the striations if you zoom into the freshly stacked image..... only poor star shapes....!!!!

EDIT:  !!!!!    Now I have looked a bit more closely  ... there does appear to be evidence of it on the 'just stacked' image 

Anybody know of what is causing this ??  Is a STACKING  artifact ??  Is it the slight movement of the star-field across the 80 subs etc??     Is it cloud passing on the individual frames ??

Can it be removed ??

Any help much appreciated.   (  please keep it simple  !!... no Pixinsight de-convoluted integrated subtraction layer vectoring suggestions..... 😖  )

 

Sean.

 

Edited January 28, 2020 by Craney

[almcl]

It looks like walking noise.  As I understand it, it's a stacking artefact resulting from a gradual movement of the guiding scope/camera relative to the imaging train.  Can be caused by mirror movement in reflectors, Viton tipped screws in the guider clamps or half a dozen other possible micron sized shifts.

Getting rid of it in post processing never fully worked for me, although clamping everything down hard helped a bit, but what really fixed it was moving to an off axis guider.

Nice Rosette, by the way. 

Edited January 28, 2020 by almcl

[Craney]

Thanks for the comment @almcl,

I'm thinking back to the set up I had operational at the time..... ( These pics were taken sometime in November 2019).   I normally have a a decent  sized refractor as the GUIDE.  (  Bresser 102mm or sometimes my 80mm Equinox).  These are both quite weighty and I had them on one of those SW guiding mount thingys....

It has often crossed my mind that these must have a max load capacity and that the axes cannot be locked.... I wonder whether this is the source of the relative drift......🤔

Thanks again...

Sean.

[tooth_dr]

Hi Sean

If I use a DSLR and don't dither I get this, but if I dither between exposures, it isn't there.  I don't know what causes it but I don't think it's to do with you saddle.

 

Adam

[Craney]

1 hour ago, tooth_dr said:

.... but if I dither between exposures, it isn't there......

Thanks Adam,

Ironically I have dithered over dithering.   I  understand the concept, but have discretely pulled the rug over it.  Getting my mount  and camera to synchronise their activities may require me to step aside and read a few manuals......yikes...  as I get older, this sensible approach to solving the problem seems to take a back seat.    (... must try harder.....)

I tend to find with all this astro -stuff,  I only make progress when faced with a real dilemma or en-passe. 

I just wish I had more time and clear nights to maintain a two steps forward / one step backward ratio !!!

Cheers for the input.   OK.  Let's try a Dither....

[michael8554]

Walking noise. 

Dithering minimises it. 

Michael 

[Craney]

12 hours ago, michael8554 said:

Walking noise. 

Dithering minimises it. 

Thanks Michael.  I'll be getting the tutorials out tonight.

 

[steppenwolf]

Dithering is a great tool for removing all sorts of noise during the stacking process and it is generally quite easy to implement in software so it is well worth getting it up and running.

[R26 oldtimer]

As others have said, it's walking noise. I first noticed it in my first guided target, then I started dithering and never saw it again.

[wimvb]

You can dither by hand, if you don't guide, but it's a royal pain. You have to sit by the mount all night. Been there, done that, never wanted the t-shirt.

Edited January 31, 2020 by wimvb

[Craney]

5 minutes ago, wimvb said:

You have to sit by the mount all night.

All Hubble'esque  with pipe and flask..........    I like my scopes, but maybe that is in the higher league of dedication.